Cancer is a disease characterized by the loss of appropriate control for cell growth. The American Cancer Society has estimated that there were in excess of 1.5 million new cases of cancer within the United Stated of America in 2010 and approximately 570,000 deaths that year estimated to be attributable to cancer. The World Health Organization has estimated that cancer was the leading cause of death globally in 2010, with the number of deaths caused by cancer growing to 12 million per year by 2030.
It has been suggested that there are 6 capabilities which need to be developed by cells in order to lead to the formation of cancerous lesions. These traits are self-sufficiency in growth signals, insensitivity to anti-growth signals, tissue invasion and metastasis, limitless replication potential, sustained angiogenesis and evasion of apoptosis. Growth signaling is required for cells to transition from a quiescent state into an active proliferative state. These signals are typically transmitted from transmembrane receptors, through signal transduction cascades involving numerous intracellular kinases, eventually resulting in changes in gene expression at the nuclear level within the cell. In recent years there has been much interest in the area of signal transduction inhibitors, particularly kinase inhibitors, and their use for the treatment of cancer. Several examples from this class of compounds have been successfully evaluated in clinical settings and are now commercially available and marketed for the treatment of specific forms of cancer e.g. imatinib tosylate (marketed as Gleevec® by Novartis for the treatment of Philadelphia chromosome-positive chronic myeloid leukemia), lapatinib ditosylate (marketed as Tykerb® by GlaxoSmithKline for the treatment of HER2 positive breast cancer in combination with other chemotherapeutic agents), sunitinib malate (marketed as Sutent® by Pfizer and approved for the treatment of renal cancer) and sorafenib (marketed as Nexavar by Bayer for the treatment of renal cancer).
In addition to the growth factor associated signaling pathways, which predominantly utilize kinase catalyzed transfer of phosphate groups as the key component of the signaling pathway, numerous other signaling pathways also exist within cells and their proper regulation is critical for maintaining correct levels of cell growth and replication. In the emerging area of cancer stem cell inhibition the Wnt, Notch and Hedgehog pathways have received much interest as potential ways in which to avoid tumor relapse and metastasis. The Wnt pathway is instrumental in embryonic development and in tissue maintenance in adults with the activity of individual components within the pathway under tight regulation. In cancer and other diseases cell signaling pathways no longer exhibit the appropriate level of control. In the case of the Wnt pathway, signal transduction is controlled by the relative stabilities of 2 proteins, axin and β-catenin. An overabundance of β-catenin leads to increased Wnt signaling and activation of associated nuclear transcription factors while excess axin results in the degradation of intracellular β-catenin and decreased signaling. Dysregulation of the canonical Wnt signaling pathway has been implicated in a range of human carcinomas such as colon cancer, hepatocellular carcinoma, endometrial ovarian cancer, pilomatricoma skin cancer, prostate cancer, melanoma and Wilms tumor.
In the canonical Wnt signaling pathway signaling is initiated by interaction of a Wnt ligand with a receptor complex containing a Frizzled family member and low-density lipoprotein receptor-related protein. This leads to the formation of a disheveled-frizzled complex and relocation of axin from the destruction complex to the cell membrane. Axin is the concentration limiting component of the destruction complex, and it is this complex which is formed with adenomatous polyposis coli proteins, casein-kinase 1α and glycogen synthase kinase 3β which is responsible for controlling intracellular levels of β-catenin. In the presence of functional destruction complex, β-catenin is sequentially phosphorylated by casein-kinase 1α and glycogen synthase kinase 3β on a conserved set of serine and threonine residues at the amino-terminus. Phosphorylation facilitates binding of β-catenin to β-transducin repeat-containing protein which then mediates ubiquitination and subsequent proteasomal degradation of β-catenin. In the absence of sufficiently elevated concentrations of the destruction complex, un-phosphorylated β-catenin is able to migrate to the cell nucleus and interact with T-cell factor proteins and convert them into potent transcriptional activators through the recruitment of co-activator proteins.
It has recently been reported that intracellular axin levels are influenced by the poly(ADP-ribose) polymerase enzyme family members tankyrase-1 and tankyrase-2 (also known as PARP5a and PARP5b) (Nature Chemical Biology 2009, 5, 100 and Nature 2009, 461, 614). Tankyrase enzymes are able to poly-ADP ribosylate (PARsylate) axin, which marks this protein for subsequent ubiquitination and proteasomal degradation. Thus, it would be expected that in the presence of an inhibitor of tankyrase catalytic activity, axin protein concentration would be increased, resulting in higher concentration of the destruction complex and decreased concentrations of unphosphorylated intracellular β-catenin and decreased Wnt signaling. An inhibitor of tankyrase-1 and -2 would also be expected to have an effect on other biological functions of the tankyrase proteins e.g. chromosome end protection (telomeres), insulin responsiveness and spindle assembly during mitosis (Biochimie 2009, 5, 100).
Therapeutics which are directed at and can correct dysregulation of the Wnt signaling pathway have been implicated in conditions such as bone density defects, coronary disease, late onset Alzheimer's disease, familial exudative vitreoretinopathy, retinal angiogenesis, tetra-amelia, Mullerian-duct regression and virilization, SERKAL syndrome, type 2 diabetes, Fuhrmann syndrome, skeletal dysplasia, focal dermal hypoplasia and neural tube defects. Although the above introduction has focused on the relevance of Wnt signaling in cancer, the Wnt signaling pathway is of fundamental importance and has potential implication in a broad range of human diseases, not necessarily limited to the examples provided above for illustrative purposes.